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I. Nuclear Magnetic Resonance

II. Continuous Wave NMR Equipment

IV. Continuous Wave NMR - The Experiment

V. NMR RF Black Box Circuit Diagram

VI. NMR Frequency Table

VII. NMR Lock In Amplifier

VIII. NMR Scope Program

IX. Pulsed NMR

X. Pulsed NMR Equipment

XI. Pulsed NMR Setup

XII. Pulsed NMR Appendices

Magnet:

In the first part of this experiment we are going to use a permanent magnet. The coils of wire wrapped around the poles of the permanent magnet are for the purpose of varying the field at a very low frequency and are needed in the later part of the experiment, where a lock-in amplifier is used.

NMR Head:

Details of the NMR head are shown in Figure 7. It is a brass box containing a radio-frequency transmitting coil positioned perpendicularly to a receiving coil into which a test tube contain a sample of protons in H₂O or other liquids is inserted. The NMR head is placed between the poles of the permanent magnet. Do not remove this head from the magnet, nor disconnect its cables. Instead, you may examine a spare NMR head on the table.

The top and bottom covers of the NMR head have 7-turn pancake coils which carry varying currents that produce a modulation field , where is 60 Hz. The amplitude is controlled from the modulation unit with gauss/amp coming from the modulating current.

NMR Box:

This black box is permanently mounted on the magnet stand and is not to be removed or disassembled; do not remove the cables either. It contains a tunable RF oscillator centered around 16.54x,xxx Mhz, a tunable receiver (left-hand knob), a diode-based detector (right-hand knob), a low-pass filter, and an amplifier for enhancing the detector output. Refer to 'NMR RF Box Circuit Diagram' in CW Appendix I for details.

The oscillator generates an RF signal that produces a magnetic field in the sample. The frequency is determined by where L and C are the combined inductance and capacitance of the coil in the NMR head, the variable capacitors in the NMR box, and the cables connecting the two. One of the capacitors tunes the circuit to hit resonance at the Larmor frequency. Because the resonant circuit includes inductors in the head and in the cables, as well as capacitors in the NMR box and the cables, touching or moving anything during measurements makes the frequency unstable and introduces lots of noise. Thus, be gentle, and keep your hands off while taking data.

The amplitude of the RF field H₁ is controlled by the DC supply voltage V₁ to the oscillator. The direction of H₁ lies in a plane perpendicular to the DC field H_o of the large permanent magnet. However, the direction relative to the axis of the receiving coil can be adjusted by rotating a copper disk on a "paddle," which controls the phase of the leakage voltage into the receiving coil. Changing the phase enables us to observe either absorption, dispersion or a mixture of the two. In this lab, we will observe adiabatic absorption or dispersion, and non-adiabatic absorption or dispersion modes. See Figures 7A, 7B, and 7C for examples of these modes.

Figure 7A: Resonance pattern corresponding to Adiabatic (Slow passage) - Absorption (~ 1.0 Molar Mn++in H2O).

Figure 7B: Resonance pattern corresponding to Adiabatic (Slow Passage) - Dispersion (~ 1.0 Molar Mn++in H2O).

Figure 7C: Resonance pattern corresponding to Non-Adiabatic passage - Absorption (~ 0.1 Molar Mn++in H2O).

Block Diagram

Refer to Figure A for the NMR Block Diagram:

The NMR head is placed in the DC field gauss of a large permanent magnet. The simplest method for observing NMR is to sweep the magnetic field through resonance at 60 Hz by superimposing a modulating H_mod on H₀. Also, it is helpful to display the NMR signal V_ac as the y-input of an oscilloscope and the 60-Hz phase shifted sine wave from the modulation unit as the x-input.

The signal from the NMR box is amplified by the PAR 113 amplifier which is used to limit the upper and lower pass frequency (typically 1 kHz and 3 Hz), thereby rejecting unwanted frequencies and improving the visibility of the desired signal. The Fluke 1953A frequency counter is used to read the NMR oscillator frequency without perturbing the oscillator.

The coils wrapped around the pole pieces of the magnet were initially used to magnetize the Alnico alloy of the poles, but are no longer needed or used for this purpose. Instead, they are used in the second part of the experiment for superimposing on H_o a small monotonically increasing field H₂(t) generated by the Function Generator (a "ramp" field). The field reaches a maximum amplitude of about 60 gauss. The ramp frequency can be as small as 0.01 Hz, thereby enabling slow, repetitive scanning in one direction through the magnetic resonance. The Lock-in amplifier (see Appendix) is used for recording the derivative of the NMR signal.

Samples:

To observe proton resonance, use the prepared set of samples of H₂O + MnCl₂4H₂O with yellow mylar tape on the top. The Mn molarity (M) ranges from M = 3.3, 1.0, 0.33, ..., to 10-5 moles of MnCl₂4H₂O per liter of solution, and gives a wide range of relaxation times. A one molar sample contains 19.7 grams of MnCl₂4H₂O in 100 cc of mixed solution. Other useful samples are glycerin with a small amount of FeCl₃, and pure H₂O. For F¹⁹ we are going to use Teflon rods. Samples should be tightly stopped and contain no air bubbles. Be careful not to drop or open the test tubes. Call for the staff if any solution comes out of the test tubes.

Digital Oscilloscope:

This experiment uses a Digital Storage oscilloscope in order to examine the weak resonance signals. See the included manual located in the reprints on how to use the digital storage scope. The output signal is also connected to the computer so that the data could be transferred to the computer and stored for later use. The instructions on how to transfer the data to the computer are located in the CW Appendix IV.

Figure A: NMR Block Diagram .Note: The Strip Chart recorder has been replaced with the computer.